Its just a simple rollover. Both graphics are the same size and the black circle is towards the top of the graphic while the blue is towards the bottom.

What/where is your website so i can take a look? I did that with Fireworks but you can do it with any text editor if you learn a little html & javascript.

If your just getting started you should check out themeforest.net. Save yourself a lot of trouble by purchasing a $10-15 website template that already has a cool navigation menu.